1. Field of the Invention
The invention relates to a steering apparatus, and more particularly, a steering apparatus in which the steering angle of a wheel can be changed with respect to the steering angle of a steering wheel.
2. Description of the Related Art
In recent years technology has been developed in which a variable transfer ratio unit, which allows the transfer ratio indicative of the steering angle of a wheel to be changed with respect to the steering angle of a steering wheel, is interposed in a steering shaft. This variable transfer ratio unit changes the transfer ratio by generating a difference of the steering amount between an input shaft and an output shaft in rotation direction. When a vehicle is provided with this kind of a variable transfer ratio unit, a differential lock mechanism is provided which selectively allows and prohibits to generate a difference of the steering amount between the output shaft and the input shaft to enable safer steering operation. One known differential lock mechanism includes a lock holder having a plurality of groove portions in the outer periphery thereof, and an engaging member that selectively engages and disengages with respect to the groove portions. It should be noted that the term “engage” in this specification refers to the engaging member entering or being anywhere in one of the groove portions. Likewise, the term “disengage” in this specification refers to the engaging member withdrawing from or no longer being anywhere in one of the groove portions.
In order to achieve suitable steering control in a steering apparatus having this kind of a variable transfer unit, Japanese Patent Application Publication No. JP-A-2004-58894, for example, proposes a vehicle steering apparatus in which the play angle when a lock arm is inserted into a concave portion is set to be less than the rotation angle of a motor when a different detection signal is output. Also, Japanese Patent Application Publication No. JP-A-2004-123083, for example, proposes a vehicle steering apparatus provided with a lock holder which rotates together with a motor and in which a plurality of first concave portions are formed in the circumferential direction on the outer periphery, and a deeper second concave portion is formed at one end portion of each of the first concave portions.
In order to appropriately change the steering angle of the wheels with respect to the steering angle of the steering wheel, it is imperative to know the positional relationship in the rotational direction between the input shaft and the output shaft when the differential lock was released. In Japanese Patent Application Publication No. JP-A-2004-58894 described above, however, there is no mention of technology for detecting which groove portion of the lock holder the lock arm is engaged with when the ignition switch is turned on, for example. In Japanese Patent Application Publication No. JP-A-2004-58894, a detection signal that is the same as the detection signal indicating that the lock arm is engaged with another groove portion, not the groove portion that the lock arm is actually engaged with, can be output from a rotation sensor. Unless it is known which groove portion of the lock holder the lock arm is engaged with, it is difficult to know the positional relationship in the rotational direction between the input shaft and the output shaft. Also, Japanese Patent Application Publication No. JP-A-2004-123083 does describe technology for detecting the groove portion that the lock arm is engaged with by detecting the width of the groove portion. However, this requires that the widths of the groove portions formed in the lock holder all be different, and the operation for that detection is also complicated.